Impeller assemblies typically include an impeller housing which is mounted on or operably connected with a central drive shaft. Attached to the shaft, within the housing, is an impeller. The impeller typically includes upper and lower cover plates and, in applications where the impeller is manufactured from pressed metal components, a vane plate located between the respective cover plates. Alternatively, the vanes of the impeller may be formed integrally with one or both cover plates. Fluid to be pumped is introduced into the impeller housing at one side thereof. The shaft rotates so as to rotate the impeller assembly thereby creating regions of high and low fluid pressure within the impeller housing and impelling fluid through the assembly.
Depending on the application of the pump, a pump can be a single-stage model i.e. having one impeller assembly, or a multi-stage model i.e. having a number of impeller assemblies in series on the same shaft passing through each of the impeller housings.
Typically, the lower cover plate of the impeller assembly incudes a central boss, formed integrally with the cover plate. The central boss defines an aperture and receives the drive shaft of the impeller assembly. The boss is typically keyed to the drive shaft so that the drive shaft directly drives the lower cover plate. The vane plate and upper cover plate have central apertures, considerably larger than the drive shaft and are located over the boss of the lower plate. The vane plate and upper cover plate are fastened to the lower cover plate e.g by welding at the vanes, gluing, or riveting. As such, the load of the entire impeller is carried by the lower cover plate as it is rotated by the drive shaft.
This distribution of load can lead to several problems when the impeller is in operation, particularly during acceleration/deceleration which may be experienced during start up or engine braking or may be due to the introduction of a foreign object into the pump housing. Because the lower cover plate only is being driven, the inertial loads of the entire impeller are transmitted to the drive feature of the lower cover plate. This plate must be accordingly stronger to resist these loads, which typically leads to a heavier, more expensive, drive feature requirement.
In the case of a laminated, pressed metal impeller, the lower plate is typically manufactured from thicker gauge material to compensate for the extra loading. In a diecast impeller, extra thickness is added locally around the drive.
Manufacture of an impeller assembly in this manner is time consuming and labour intensive, requiring, in the case of welding, numerous spot welds between the lower cover plate and the vane plate, and between the vane plate and the upper cover plate. The plates must be securely fixed together so as to prevent slippage and fluid flow between the plates.
In the case of plastic impellers, welding can introduce variation in the axial length of the impeller assembly. With too much welding, this length is reduced, leading to a reduction in the impeller flow output. With insufficient welding, the impeller axial length will be increased, potentially leading to overloading of the drive motor.
Mechanical fastening, in the form of riveting can lead to failure due to fretting and is also known to lead to corrosion problems, as materials are more prone to stress induced corrosion after riveting.
Permanent fastening of the impeller components also prevents easy dismantling and replacement of individual components in the assembly if they become worn or faulty.
The above disadvantages are of course amplified when the pump is a multi-stage model. In particular, variation in the axial length of individual assemblies is multiplied, leading to fitment problems on mating seal components, in addition to the performance variation described previously.
It is therefore an object of the invention to provide an impeller assembly that at least in part alleviates one or more of the above disadvantages.